THE BLOOD. 87 



the blood-liquid by any of the above-mentioned means (see page 66) 

 the blood becomes transparent and acts then like a "lacfarbe." 

 Less light is now reflected from its interior, and this latter blood 

 is therefore darker in thicker layers. On the addition of salt 

 solutions to the blood-corpuscles they shrink and more light is 

 reflected and the color appears lighter. A great abundance of red 

 corpuscles makes the blood darker, while by diluting with serum or 

 by a greater abundance of white corpuscles the blood becomes 

 lighter in appearance. The different colors of arterial and of ven- 

 ous blood depend on the varying quantity of gas contained in these 

 two varieties of blood, or better on the different amounts of oxy- 

 haemoglobin and haemoglobin they contain. The reason for the 

 different colors of these two varieties of blood has been attributed 

 in part to the unequal forms of the blood-corpuscles. In the arterial 

 blood the blood-corpuscles are considered as more biconcave and 

 therefore reflect the light more than in the venous blood (HAKLESS). 

 These statements do not coincide with later investigations. 



The most striking property of blood consists in its coagulating 

 within a shorter or longer time, but as a rule very shortly after 

 leaving the vein. Different kinds of blood coagulate with vary- 

 ing rapidity ; in human blood the first marked sign of coagulation 

 is seen in 2-3 minutes, and within 7-8 minutes the blood is thor- 

 oughly converted into a gelatinous mass. If the blood is allowed 

 to coagulate slowly, the red corpuscles have time to settle more or 

 less before the coagulation, and the blood-clot then shows an upper, 

 large, yellowish-gray or reddish -gray layer consisting of fibrin en- 

 closing chiefly colorless corpuscles. This layer has been called 

 crusta inflammatoria or phlogistica, because it has been especially 

 observed in inflammatory processes, and is considered one of the 

 characteristics of them. This crusta or "buffy coat" is not char- 

 acteristic of any special disease, and it occurs chiefly when the blood 

 coagulates slowly or when the blood-corpuscles settle more quickly 

 than usual. A buffy coat has been observed often in the slow 

 coagulation of equine blood. The blood from the capillaries is not 

 supposed to have the power of coagulating. 



Coagulation is retarded by cooling, by diminishing the oxygen 

 and increasing the amount of carbon dioxide, which is the reason 

 that venous blood and to a much higher degree blood after asphyx- 



